An article of relatively pure fused silica, and a furnace and method for producing the article.
Relatively pure metal oxides are produced by thermal decomposition of precursors and deposition of the resulting oxides. The precursor may take the form of a vapor, or may be carried by a vapor. It may be decomposed by either flame hydrolysis or pyrolysis.
One such process is production of fused silica by hydrolysis or pyrolysis of silicon tetrachloride. Early patents disclosing such processes for producing silica are U.S. Pat. No. 2,239,551 (Nordberg) and U.S. Pat. No. 2,272,342 (Hyde). A commercial application of flame hydrolysis involves forming and depositing particles of fused silica to form large bodies (boules). Such boules may be used individually, or may be finished and integrated together into large optical bodies, such as telescope mirrors. In this procedure, SiCl4 is hydrolyzed, and the hydrolyzed vapor is passed into a flame to form molten particles of fused silica. The particles are continuously deposited on a bait, or in a crucible, known as a cup, to form a boule.
A serious drawback in this process has been the need to dispose of the HCl by-product in an environmentally safe manner. Accordingly, it has been proposed, in U.S. Pat. No. 5,043,002 (Dobbins et al.), to employ a halide-free, silicon-containing compound as a substitute for SiCl4. In particular, the patent proposes using a polymethylsiloxane, such as octamethylcyclotetrasiloxane, to provide the vaporous reactant for the hydrolysis or pyrolysis process.
In order to introduce a substitute precursor, it is, of course, critically necessary to avoid any significant change in the properties of the fused silica product. Unfortunately, the substitution proposed by the Dobbins et al. patent did lead to significant property changes. One such change was a reduction in UV transmission properties. Another was development of fluorescence in the glass that increased when the glass was exposed to short wavelength radiation.
Studies revealed that a factor in the transmission loss was sodium ion content in the glass. U.S. Pat. Nos. 5,332,702 and 5,395,413 (Sempolinski et al.) describe remedial measures taken to reduce the sodium ion content. Essentially, these measures constituted providing a purer zircon refractory for use in constructing a furnace in which the fused silica was deposited to form a boule. In particular, it was found necessary to use dispersants, binders and water relatively free of sodium ions in producing zircon refractory components for the furnace.
An improved product was obtained by adopting the practices prescribed in the Sempolinski et al. patents. However, use of the fused silica in certain applications made it apparent that further improvements were necessary to meet the critical requirements of these applications. One such application is lenses designed for transmission of very short UV wavelength radiation from an excimer type laser. This laser emits radiation at about 193 nm and 248 nm wavelengths.
It was found that lenses produced from available fused silica did not provide acceptable transmission of the short wavelength radiation and exhibited an undesirable fluorescence. Both of these conditions tend to become worse with service time. The loss of transmission, or darkening of the glass, is commonly referred to as UV absorption damage.
It is a primary purpose of the present invention to provide a fused silica material that alleviates these problems. Another purpose is to provide an improved fused silica glass for lenses used with lasers, especially for microlithographic work. A further purpose is to provide an improved furnace for collection of the fused silica in the form of a boule. A still further purpose is to provide a method of achieving the improved collecting furnace and glass produced therein.
The invention includes a method of making a fused silica glass. The method includes providing a silica feedstock, and providing a furnace crown and cup consisting essentially of aluminum dioxide, with the crown covering a consolidating non-porous fused silica glass mass. The method further includes delivering the silica feedstock to reaction site burners mounted within the aluminum dioxide crown wherein the silica feedstock is converted into silica particles which are deposited and consolidated into the fused silica glass mass.
The invention further includes a fused silica glass furnace for converting a non-silica fluid silica precursor feedstock into a fused silica glass, the fused silica glass furnace having a contained furnace interior. The contained furnace interior has a maximum furnace operation temperature MFOT. The furnace interior is comprised of a conversion deposition consolidation site where the precursor feedstock is converted into silica soot and the converted silica soot is then deposited and consolidated into a fused silica glass. The furnace interior is contained and insulated by aluminum dioxide refractory bricks with the aluminum dioxide refractory bricks having a fired temperature FT, where FT greater than 1650xc2x0 C., and the aluminum dioxide refractory bricks consist essentially of Al and O.
An embodiment of the present invention resides in an improved method of producing a fused silica body by introducing a silicon-containing compound into a lame to form molten silica particles and collecting those particles in the form of a fused silica body in a furnace constructed of refractory materials, the improvement comprising constructing at least a portion of the furnace from refractory materials that have been exposed to a reactive, halogen-containing gas to react with and thereby cleanse the refractory of contaminating metals.
A further aspect of the invention resides in a relatively pure fused silica material in which the fused silica has a transmittance value of at least 99.5% for 248 nm radiation, a transmittance value of at least 98% for 193 nm radiation, at least a substantial portion of the body has an acceptable fluorescence level when exposed to such radiation, and the fused silica material has a content of contaminating metal ions less than 100 ppb.
The invention further resides in a refractory furnace for collecting molten silica particles in the form of a solid body, at least a portion of the furnace being constructed of a refractory that contains mobile metal contaminants in an amount less than 300 ppm.